FIG. 6 shows a conventional drive device for driving an objective lens to preform the focusing servo and tracking servo operations. This conventional drive device is disclosed in U.S. Pat. No. 4,861,138.
A magnetic flux designated by reference numeral 6 is produced in the drive device of the above construction, and when electric current is supplied to the tracking coil 3, an electromagnetic force is exerted in the direction of the arrow T on those two portions of the tracking coil 3 facing the north pole and south pole surfaces, respectively. The objective lens 1 is driven, together with the holder member 2, by this electromagnetic force to thereby effect the tracking servo.
In the above drive in which a magnetic flux is produced between the two pole surfaces of opposite polarity disposed adjacent each other in a plane, the magnetic circuit is very thin, and therefore the overall construction of the device can be compact. However, the drive device requires a large and relatively expensive magnet, and therefore the overall cost is high. The magnet of such a construction is produced by magnetizing a flat plate of magnetic material using a magnetizer having two pole surfaces and cannot be perfectly magnetized in the vicinity of the boundary therebetween. To compensate for this, the magnet must be increased in size. This increases the size of the tracking coil, and offers (a problem to be solved in) an obstacle to achieving a compact overall construction of the device.